Always include the adb version in the log.

"Are you running the right version?" seems to be a common cause of debugging
confusion.

Change-Id: I4a4ba95e876dafc05e515058ca059ea370273f78
diff --git a/adb/adb.h b/adb/adb.h
index b0e53f0..016cb11 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -46,6 +46,8 @@
 #define ADB_VERSION_MAJOR 1
 #define ADB_VERSION_MINOR 0
 
+std::string adb_version();
+
 // Increment this when we want to force users to start a new adb server.
 #define ADB_SERVER_VERSION 32